Cisco DHCP scope and DNS server addressing

Discussion in 'Cisco' started by seanovision, Jun 22, 2007.

  1. seanovision

    seanovision Guest

    Same setup in a previous post, but different issue. My 2611 has eth0 se
    up as a DHCP client to my cable modem and it does receive the DNS serve
    addresses that is is supposed to as evidenced by show hosts (and they'r
    definitely not set static).

    Eth1 is set up as a DHCP server for the LAN. It's handing out IPs an
    the default gateway information, but not the DNS server addresses.

    I tried setting eth1's address as the DNS server in the scope, an
    eth0's, and I've tried leaving it as blank,...... but none will let m
    clients resolve names.

    And I don't want to set the DNS server addresses that eth0 recieve
    from the cable modem into the LAN scope, since the ISP can change thos
    DNS server addresses at any time.

    Any suggestions

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    seanovision, Jun 22, 2007
    #1
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  2. seanovision <> writes:
    >Same setup in a previous post, but different issue. My 2611 has eth0 set
    >up as a DHCP client to my cable modem and it does receive the DNS server
    >addresses that is is supposed to as evidenced by show hosts (and they're
    >definitely not set static).


    >Eth1 is set up as a DHCP server for the LAN. It's handing out IPs and
    >the default gateway information, but not the DNS server addresses.

    ....
    >And I don't want to set the DNS server addresses that eth0 recieves
    >from the cable modem into the LAN scope, since the ISP can change those
    >DNS server addresses at any time.



    I don't think its possible to take the DNS servers received by the
    DHCP client, and rebroadcast them in the DHCP packets sent out by the
    DHCP server process on IOS.

    BUT, its highly uncommon for an ISP to rotate DNS server IP addresses,
    those usually are pretty well set in stone for clients to use. Unless
    they have a fairly short lease time, they could have clients connected
    for sometime using the values they handed out some time ago.

    If you are really concerned about the ISP really truely dropping DNS
    resolver servers constantly and don't want to hard-code them into
    the DHCP server process on IOS, use public DNS servers that are
    set-in-stone like OpenDNS's servers.


    If anything, most of the really big ISP's anycast the same IPs for
    their DNS cache resolvers across their whole network such that the
    same DNS server IP address will work consistantly and network wide,
    even if that server is down, the next geographically closest in the
    cluster will anycast for that IP address if one goes down.
     
    Doug McIntyre, Jun 22, 2007
    #2
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  3. Martin Gallagher, Jun 22, 2007
    #3
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